1. Field of the Invention
This invention relates to a phosphor for a cathode ray tube, in particular, a phosphor for a cathode ray tube used as a light source for color copying or color printing, or, in particular, a phosphor having uniform and bright-light emission energy strength, a cathode ray tube using said phosphor and a color copying (or printing) unit using said cathode ray tube.
2. Related art
In an image processing system for a color copying machine, a color printer, etc., it has been general practice to obtain a copy image by imagewise exposing a photosensitized material formed on a photosensitive drum, etc., to form an electrostatic latent image, developing/transfering the electrostatic latent image with a toner, and repeating this procedure for color elements three times. However, a new method called cycolor has been recently developed.
This method is carried out by exposing a special film coated with numerous ultrafine particles reactive to red, green and blue lights, transfering a resultant image to a copying paper and fixing it, i.e. the development and transfer are carried out in one process (S. Lu et. al., Laser-Addressed Liquid Crystal Light Modulators for Color Electronic Imaging with Mead Microencapsuled Paper, 1987 Society for Information Display International Symposium Digest of Technical Papers, pp. 367-370, May 1987).
The light source for the above cycolor method is required to fulfill the following three requirements.
(1) The light source is required to have a constant light emission energy strength, a constant distribution of the energy strength and a constant-height peak in each of regions suitable for spectral sensitivity in regions of blue, green and red of a special film.
(2) The light source is required to have a light emission energy strength and peak each in regions suitable for blue, green and red filters.
(3) The light source is required not to cause non-uniform light emission.
When a laser beam is used as a light source, the compaction of a color copying (or printing) unit is limited. Therefore, the use of a cathode ray tube is taken into consideration as a substitute light source. As a cathode ray tube to fulfill the above requirements (1) and (2), there is a cathode ray tube having a phosphor surface formed by mixing blue, green and red-emitting phosphors and coating the mixture. For example, one cathode ray tube has a phosphor surfacer formed of a mixture of equal amounts of ZnS:Ag.Al as a blue-emitting phosphor, Y.sub.3 Al.sub.5 O.sub.12 :Tb as a green-emitting phosphor and Y.sub.2 O.sub.3 :Eu as a red-emitting phosphor, and such a cathode ray tube has a light emission energy strength as shown in FIG. 3. This cathode ray tube nearly fulfills the above requirements (1) and (2) concerning the light emission energy strength, distribution and peak, etc., in the blue region (wavelength 400-500 nm), green region (wavelength 520-580 nm) and red region (wavelength 600-700 nm). However, when a color copy was made by using this cathode ray tube, it was found that this cathode ray tube has defects that an image non-uniformity corresponding to a non-uniform mixture of the phosphors is caused and that a reduction in resolution occurs. This phenomenon could not be removed even if the mixture was highly homogenized to improve the mixture uniformity. And, it has been found that this phenomenon is caused by uneven distributions of the particle sizes and aggregates of the phosphors, and that no cathode ray tube can be put to practical use as far as a mixture of phosphors is used.
Then, a study was made of a cathode ray tube having a phosphor surface formed of a single phosphor having a light emission energy distribution in each color region. As a phosphor having a light emission energy distribution in blue, green and red regions, there are white-emitting phosphors, and a terbium-activated, yttrium oxysulfide (Y.sub.2 O.sub.2 SL:Tb) phosphor having a JEDEC register No. P45 is known. When a cathode ray tube having a phosphor surface formed by coating this phosphor was used to make color copies, the resultant image quality showed that the image non-uniformity caused by the use of the foregoing mixture of phosphors was overcome, and that the resolution was also good. In this case, however, the light emission energy distribution was as shown in FIG. 2, and there was imbalance in the distribution of light emission energy strength, i.e. the strength in the red region was extremely low. It has thus been found that such a cathode ray tube is practically undesirable. In this Y.sub.2 O.sub.2 S:Tb phosphor, the light emission energy distribution can be changed by increasing or decreasing the concentration of Tb, an activator. However, even if such a change is possible, the light emission strength in the red region cannot be improved.
Further, other single white-emitting phosphors which have been commercially available were studied. However, none of such phosphors could give a cathode ray tube having satisfactory properties.